This invention is directed to a method of dispersing hydrophobic particles in aqueous suspensions using ethylene oxide-propylene oxide block copolymers and to aqueous compositions comprising the ethylene oxide-propylene oxide block copolymers.
The deposition of organic contaminants on process equipment, screens, and containment vessels in papermaking significantly reduces process efficiency and paper quality. Deposits on machine wires, felts, foils, headbox surfaces, screens, and instruments require costly downtime for cleaning to avoid the problems associated with poor process control, reduced throughput, and substandard sheet properties. These contaminants are generically referred to in the paper industry as either xe2x80x9cpitchxe2x80x9d or xe2x80x9cstickiesxe2x80x9d.
Pitch deposits generally originate from natural resins (fatty acids, rosin acids, sterols, etc), resin salts, coating binders, or sizing agents found in the pulp (Journal of Pulp and Paper Science, 22:11,431-439, 1996). Pitch may also contain inorganic materials such as talc, calcium carbonate, or titanium dioxide. Stickies generally refers to the hydrophobic substances used in the manufacture of paper such as pressure sensitive or contact adhesives such as styrene butadiene rubber, hot melt adhesives such as ethylene vinyl acetate and thermoplastics inks (styrene acrylate, polyvinyl butyral) that can form deposits when reintroduced in recycled fiber systems. Other common hydrophobic contaminants found in recycle applications include wax, which originates primarily from wax-coated old corrugated containers, and polyisoprene.
Recycled fiber refers to secondary fibers which are repulped to provide the papermaking furnish with raw material for the production of new papers. The secondary fibers may be either pre-consumer or post-consumer paper material that is suitable for use in the production of paper products. Sources of secondary fiber may include old newspaper (ONP), old corrugated containers (OCC), mixed office waste (MOW), computer printout (CPO), ledger, etc. These once-processed papers contain various types of adhesives (pressure sensitive, hot melts, etc.), inks, and coating binders. Coating binders can include the so-called xe2x80x98white pitchxe2x80x99 associated with the repulping of polyvinyl acetate and styrene butadiene coated broke.
Pitch and stickies are hydrophobic in nature and thus quite unstable as colloids in aqueous papermaking environments, thereby facilitating their deposition. The major problems arising from deposition are as follows: (1) reduced throughput due to plugging of forming fabrics and press felts (2) sheet holes or paper breaks due to large deposits breaking loose from the equipment, and (3) reduced sheet quality due to contaminants incorporated in the final sheet.
To deal with the problem of stickies, a number of strategies have been employed by papermakers. These strategies include mechanical and chemical means to either remove or detackify the contaminants. Mechanical means of removing the contaminants include slotted pressure screens, hydrocyclones, and cleaners of various types. Also, thermal/mechanical dispersion units are employed to break the contaminants into micron sized particles, which are then difficult to detect in the final sheet. Despite these techniques 100% removal of contaminants by mechanical means cannot be accomplished.
Chemically, several approaches are taken, including passivation or detackification of the sticky contaminant surfaces. Detackification means the process of reducing the rate of detrimental deposits originating from pitch or stickies. Dispersing is one route of detackification that improves the colloidal stability of contaminant particles in aqueous systems. Common detackification agents include inorganic materials such as talc and zirconium compounds, and organic materials such as polyvinyl alcohol. In addition, various dispersants may be used to prevent the contaminants from agglomerating. The small, dispersed contaminants may be fixed to the paper sheet by the use of cationic polymers. Finally, if the contaminant problem becomes severe, solvents may be used to wash and remove the materials from machine surfaces and clothing.
Previous studies show that a variety of nonionic or anionic polymer/surfactant types can be effective in reducing the surface tension, adhesion tension, and contact angles of aqueous solutions in contact with a polyester surface (Pulp and Paper Canada, 94:10, 17-23, (1993)). These studies, however, relate to the deposition tendency of hydrophobic particles onto surfaces rather than dispersancy of the stickies in aqueous environments.
Similar peel test data has been used extensively to demonstrate the detackification properties of chemical additives such as polyalkylene oxide/vinyl acetate copolymers (U.S. Pat. No. 5,266,166), terphthalate gycol terpolymers (U.S. Pat. No. 5,415,739), methyl cellulose derivatives (U.S. Pat. No. 4,781,794), and polyvinyl alcohol (U.S. Pat. No. 4,956,051).
Peel test data has also been used to show that polyelectrolyte complexes comprised of oppositely charged materials are also useful detackification agents (TAPPI Journal, 81:6, 143-151, (1998), U.S. Pat. No. 5,292,403). While the peel test data has meaningful applications in the paper industry, it fails to accurately predict the relative adhesion properties between aqueous hydrophobic particles in the presence of chemical additives.
More recently, atomic force microscopy (AFM) has been developed as a tool for measuring the microscopic adhesion forces between hydrophobic particles of colloidal dimension in aqueous solutions (J. Colloid and Interface Sci., 185, 363-370, (1997)). This technique has even been applied to adhesion forces between hydrophobic particles and paper (TAPPI J., 82:5, 172-174, (1999)). The AFM measurements are a much better simulation of interparticle collisions in aqueous solution than the peel tester. The industrial standard peel tester applies forces of xcx9c800 kN/m2 over xcx9c100 cm2 for 2 minutes compared to the 5 kN/m2 forces applied over xcx9c30 xcexcm2 for milliseconds in AFM measurements. The surface preparation, applied forces, surface areas, and adhesion times are vastly different, with the AFM protocol more closely resembling interparticle collisions in all aspects. Therefore, AFM results will have greater applicability to aqueous dispersancy of hydrophobic particles than the peel tester data, which may have greater applicability in dryer section deposition problems.
International Patent Application No. PCT/US98/05261 discloses a method of removing pitch and stickies from papermaking fiber comprising treating the fiber with a composition comprising ethylene oxide-propylene oxide block copolymers, an organic solvent and a surfactant and then mechanically separating the agglomerated stickies and pitch from the papermaking fiber.
A method of dispersing wax during the recycling of coated paper products using a plasticizer and an ethylene-oxide/propylene oxide block copolymer is disclosed in commonly-assigned Ser. Nos. 09/039,863 and 09/258,627.
Using AFM adhesion force measurements, we unexpectedly discovered a series of block copolymers of ethylene oxide and propylene oxide (EO/PO block copolymers) that significantly outperform the traditional industry standard chemical treatment programs for detackification.
Although the surface activity of EO/PO block copolymers has been previously reported via contact angle and surface tension measurements, (Pulp and Paper Canada, 94:10, 17-23, (1993)) their effectiveness in reducing adhesion forces between hydrophobic particles in aqueous medium has never been demonstrated until now. This property is extremely valuable for aiding in the dispersion of hydrophobic particle contaminants in recycled paper applications.
Thus, the EO/PO block copolymers described herein are highly effective and efficient additives for dispersing hydrophobic particles in aqueous suspensions. The addition of EO/PO block copolymers to process waters in recycled fiber papermaking applications improves papermaking efficiencies by reducing deposits originating from pitch or stickies.
Accordingly, in its principal embodiment, this invention is directed to a method of dispersing hydrophobic particles in aqueous suspensions comprising adding to the suspension an effective amount of an ethylene oxide-propylene oxide block copolymer, wherein the copolymer has a molecular weight of from about 1,000 to about 50,000 and a HLB of from about 1 to about 4.
The EO-PO block copolymers of this invention consist of repeating ethylene oxide (EO) and propylene oxide (PO) polymer blocks and have a HLB of from about 1 to about 4. These low-HLB EO/PO block copolymers are widely known and commercially available from a variety of sources for use as defoaming and antifoaming agents in a number of applications. See xe2x80x9cPLURONIC(copyright) and TETRONIC(copyright) Surfactantsxe2x80x9d, BASF Performance Chemicals, BASF Corporation, 1996.
The EO/PO block copolymers of this invention have formulas 1-4 below where a-v are independently integers of from 1 to about 1000 such that EO/PO block copolymer has a molecular weight of from about 1,000 to about 50,000. 
EO/PO block copolymers of formula (1)-(4) are also referred to as EO/PO/EO block copolymers, PO/EO/PO block copolymers, EO/PO/EO branched, block copolymers, and PO/EO/PO branched, block copolymers, respectively.
In a preferred aspect of this invention, the ethylene oxide-propylene oxide block copolymer comprises from about 10 percent to about 80% ethylene oxide units by weight and has a molecular weight of from about 2,000 to about 20,000.
In another preferred aspect, the ethylene oxide-propylene oxide block copolymer comprises about 10 percent ethylene oxide units by weight and has a molecular weight of about 3800 and a HLB of about 1.
In another preferred aspect, the EO-PO block copolymer is added to a papermaking slurry.
In another preferred aspect, the papermaking slurry comprises recycled fiber.
In another preferred aspect, this invention is directed to an aqueous composition comprising an ethylene oxide-propylene oxide block copolymer and one or more surfactants wherein the ethylene oxide-propylene oxide block copolymer has a molecular weight of from about 1,000 to about 50,000 and a HLB of from about 1 to about 4 and the surfactant has a HLB greater than about 18.
The surfactant is added to solubilize the EO-PO block copolymer in water. Generally any surfactant having a HLB greater than 18 is suitable. Representative surfactants include non-ionic surfactants such as alcohol ethoxylates (e.g. Tergitol(copyright) 15-S-40, available from Union Carbide Corp., Danbury, Conn.), anionic surfactants such as TEA dodecylbenzene sulfonate (e.g. Norfox T-60, available from Norman, Fox and Co., Vernon, Calif.), sodium lauryl sulfate(e.g. Rhodapon LSB, available from Rhxc3x4ne-Poulenc, North American Chem. Surfactants and Specialties, Cranburry, N.J.), and the like, and cationic surfactants such as a fatty quaternary ammonium derivatives (e.g. Atlas G-265, available from ICI Surfactants, Wilmington, Del.).
When acidic surfactants such as dodecylbenzenesulfonic acid are used, the composition may contain additives such as triethanolamine, sodium hydroxide, and the like to adjust the pH of the composition. In a preferred aspect, the aqueous composition as a pH of about 7.
Additional medium HLB surfactants (e.g. ethoxylated alcohols having a HLB range from 7-15) may be added to increase the HLB range of the composition. Additional surfactants may also be included in the composition to address issues of foaming (e.g. silicon based surfactants, sorbitan oleates, glycerol oleates, etc.) These additional surfactants are typically added at a concentration of up to 10 percent.
In another preferred aspect, the weight ratio of ethylene oxide-propylene oxide block copolymer to surfactant is from about 5:1 to about 1:1.
In another preferred aspect, the surfactant is sodium dodecylbenzenesulfonate.
In another preferred aspect, the weight ratio of ethylene oxide-propylene oxide block copolymer to surfactant is about 1:1.
In another preferred aspect, this invention is directed to a method of dispersing hydrophobic particles in aqueous suspensions comprising adding to the suspension an effective amount of an aqueous composition comprising an ethylene oxide-propylene oxide block copolymer and one or more surfactants wherein the ethylene oxide-propylene oxide block copolymer has a molecular weight of from about 1,000 to about 50,000 and a HLB of from about 1 to about 4 and the surfactant has a HLB greater than about 18.
In another preferred aspect, the aqueous suspension is a papermaking slurry.
The EO-PO block copolymer may be added at any point in the wet end of the papermaking process where xe2x80x9cwet endxe2x80x9d means the parts of the papermaking process occurring before the final press and dryer section. Preferably the EO-PO block copolymer is added to the papermaking slurry at a location that allows intimate contact between the chemical treatment and the paper stock, e.g. pulper, machine chest, refiners, etc. The dosage of the product will depend on the level of hydrophobic contaminants present in the fiber stock. The dosage is usually in the range of 0.05%-1% based on dry fiber.